In order to steady the roof of a mine adit, drift or gallery against collapse, substantially vertical bores may be drilled from below into the overhead rock and may be stuffed with a sealant or bonding agent designed to anchor a set of bolts firmly in position therein, each of these bolts having a threaded lower extremity which projects from the roof surface and traverses a perforated retaining element such as a large washer which bears upon that surface under pressure of a nut screwed onto this threaded extremity. Bonding agents generally used for this purpose are of a multicomponent type, their several components including a hardenable mass such as a thermosetting resin, a curing agent therefor and, if desired, a catalyst and an accelerator. At least the hardenable mass, e.g. an epoxy resin, and the curing agent, e.g. an amine, must be separately packaged in frangible envelopes initially preventing their interaction. (See, for example, U.S. Pat. No. 2,982,396 and British Pat. No. 765,082.) Upon insertion of the bolt into a bore occupied by adhesive components so packaged, the envelopes are pierced and their contents intermingle so as to react with one another. To ensure a dependable bond, the mixture should be homogenized by vigorous agitation before setting. For this purpose, the inserted part of the bolt may be provided with a roughened surface, preferably in the form of a helicoidal groove, which upon rotation of the bolt has a stirring effect and which after the setting of the adhesive serves for more effective bonding.
If the lower end of the bolt has a polygonal head designed to facilitate its rotation, separate tools must be used to install each reinforcement, namely a first socket wrench fitting that head and a second socket wrench mating with the screw for tightening same against the retaining element after the bolt has been firmly emplaced. While the tools can be driven automatically, their interchange between the stirring and tightening steps is somewhat laborious and requires the intervention of an operator who may be exposed to the danger of falling rocks from a not yet consolidated roof portion.
Without such preliminary rotation, a reliable anchorage of the bolt in the overhead rock is difficult to attain even if the amount of adhesive is sufficient to fill the entire bore around the inserted bolt. If the lower end of the bolt is used to support a load, e.g. to serve as a point of attachment for a cable or the like, relative shifts between the bolt and the rock at that lower end tend to weaken the bond in that region so that the rock is liable to split around the bore, thereby seriously impairing the stability of the reinforcement.